1. Field of the Invention
The invention relates to a method and system for controlling transmission and reception of signals traveling through multiple optical fibers. Particularly, this invention relates a method and apparatus for setting an apparent optical length of multiple fibers transmitting an optical signal of a given wavelength, wherein the apparent optical length being a function of a refractive index of each individual fiber and its actual length.
2. Description of the Related Art
Communication via beams of light traveling over thin glass fibers forms a major part of numerous infrastructures for national communication networks. Fibers find their greatest use in telephone networks, local-area networks, and cable television networks. They are also useful for short data links, closed circuit video and audio links, and elsewhere. Typically, the fiber-optic networks are digital and often transmit hundreds of thousands of channels along a single cable which may contain up to hundreds of individual fibers. Fibers are practical for a range of path lengths from under a meter to as long as required on the earth's surface and beneath its oceans.
Optical fibers are thin, long cylindrical structures which support light propagation through total internal reflection. An individual optical fiber consists of an inner core and an outer cladding typically made of silica glass, although other materials like plastics are sometimes used. Optical fiber, being a physical medium, is subjected to perturbation of one kind or the other at all times. It therefore experiences geometrical (size, shape) and optical (refractive index, mode conversion) changes to a larger or lesser extent depending upon the nature and the magnitude of the perturbation. In communication applications, such effects can negatively affect signal transmission and reception, and therefore it is desirable to minimize these effects.
Since light is characterized by amplitude (intensity), phase, frequency and polarization, any one or more of these parameters may undergo a change. The usefulness of a fiber optic sensor therefore depends upon the magnitude of this change and the ability to measure and quantify the same reliably and accurately.
A typical point-to-point fiber optical communication system operates to deliver light signals generated by a transmitter, which includes, for example, a modulator and a light source, to a receiver via multiple optical fibers. An input signal, such as a light wave of a given length, traveling through a long fiber arrives at the end of the fiber with a phase angle related to the apparent length of fiber, which is a function of an actual length of the fiber and its refractive index. Accordingly, if the phase angle varies from fiber to fiber, the output signals at downstream ends of individual fibers will optically interfere with one another even if a fixed, uniform input signal or in-phase input optical signals is/are launched into upstream ends of the fibers.
In many applications, it is important to control the phase relationships between the individual fiber lengths that are distributed over large distances and are subject to a variety of environmental conditions affecting the length of any given fiber. A non-uniform length of the fibers results in variation of the phase shift at downstream ends of the fibers, which can be significant enough to impair the quality of a transmitted signal(s). It is desirable that input signals transmitted through multiple fibers will emerge out of downstream ends of these fibers substantially simultaneously, e.g., the input signals will arrive at the downstream ends with a substantially uniform phase angle and, thus, optically coincide with another. Alternatively, if desired, the phase angle of the transmitted signals emerging out of the downstream ends can be varied, so that the transmitted signals will controllably interfere with one another.
It is well known that the arrival phase angle ι with which a transmitted signal emerges out of a downstream end of a fiber can be adjusted by changing an actual length of the fiber and/or a refractive index of this fiber. The refractive index is defined as n=Vvac/V, where Vvac is the speed of light in vacuum (essentially equal to that in air) and V is the speed of light.
Referring to FIG. 1, optical fibers 10 and 12 have length L1 and length L2, respectively. L1 is not equal to L2. If optical carrier wave signals are each injected into individual fibers 10, 12 in phase, then they will arrive at downstream ends 16, 18, respectively, as output signals 3 and 4 with relative phase relationships based on the fiber lengths L1 and L2 and the initial signal phase relationships of the optical signals 1 and 2. For simplicity of description, assume that the signals 1 and 2 originate in-phase at point A, and then the signals will arrive at receiving ends 16, 18 of the fibers 10, 12, respectively, as output signals 3 and 4 with relative phase relationships based exclusively on the fiber lengths. The arrival phase angle of the signal at the end of the optical fiber can be determined in accordance with the following formula:ι=2πnL/□0  (1)where n is the index of refraction and □0 is the wavelength of light in the vacuum and L is fiber length. Based on equation (1), the fiber length can be determined in accordance with the following equation:L=ι□0/2πn  (2)
After differentiating and simplifying, a phase difference ι between ι, and ι2, as shown in FIG. 7, is defined by the following equation:ι=2πnL/□0  (3)wherein L is the length difference L2−L1.
The time of arrival of the light signals 1 and 2 at receiving ends 16, 18 of the optical fibers 10, 12, respectively, varies, e.g., the signals will emerge out of the downstream end 18 later than out of the downstream end 16 because the optical fiber 12 is the longest. The arrival phase angles ι1, ι2 are different too.
In current practice, controlling phase relationships between signals traveling through multiple fibers, that is a phase relationship between arrival phase angles i1, i2, requires trimming each fiber to a substantially uniform actual length. In the simplest implementation of this practice, initially each fiber is made the same length within a tolerance of an acceptable phase error. Even if only one of a multiplicity of fibers is cut so that its length differs from a relatively uniform length of all other fibers, then either its length will have to be modified by the addition of an extra length of fiber to be spliced to the last fiber and re-trimmed, or all of the previously cut fibers will require re-trimming. Trimming fibers, particularly relatively long fibers, is difficult due to limitations of making the total fiber length measurement. Furthermore, the precision available in instruments used to measure fiber length also negatively affects the effort needed to provide relatively uniform lengths of multiple optical fibers.
It is, therefore, desirable to provide a method of setting an apparent optical length of multiple fibers by controllably affecting a refractive index of individual optical fibers. Also, an apparatus for setting an apparent optical length of multiple fibers utilizing the inventive method is desirable as well.